I found how to demonstrate the Taylor Series with integral remainder.
$$ f(x) = \sum_{k=0}^n \Biggl[\frac{f^{(k)}(a)}{k!}(x-a)^k \Biggr] + \int_a^x \frac{f^{(n +1)}(t)}{n!}(x-t)^n dt $$
But I do you pass to this formula to the formula with the $o(x^n)$ term ?
$$ f(x) = \sum_{k=0}^n \Biggl[\frac{f^{(k)}(a)}{k!}(x-a)^k \Biggr] + o_a((x-a)^n) $$
How is it possible to demonstrate it ?
Thank you for your help.